Add OperatorAdapterViewOnItemClick to observe OnItemClick events in AdapterViews. Resolve #19.

- new `OperatorAdapterViewOnItemClick` to handle onItemClick events from AdapterViews;
- separate `OnItemClickEvent` class to observe it;
- corresponding `itemClicks` method at `ViewObservable`;
- tests for ListView and GridView, also with multiple subscriptions.
The point here is to make all this complicity transparent to the end developers. I don't think it's a good idea to force them to use `ConnectableObservable` instead of providing the same functionality out of the box.

By the way, `OperatorAdapterViewOnItemClick` is not a real `Operator`, since it implements `OnSubscribe` interface, but not the `Operator` one.  Probably we should follow the conventions used in `RxJava` and name it something like `OnSubscribeViewOnItemClick` (and rename all other "operators" as well).